Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Flaw detection system using microwaves
The invention relates generally to systems for the
detection of flaws in non-conducting materials such as e.g.
lumber. Defects in lumber, for instance knots, can be
readily detected using a microwave technique. The knot
brings about change in the radiatior passing through
compared with the equivalent value measured in a knot-free
area of the same lumber.
The method of the present invention is based on the use
of two microwave transmitting means which are positioned to
transmit radiation of equal field intensity, but with a
phase shift of 180 between them, to one side of the
lumber. In case of knot-free lumber there is a null in the
microwave field pattern at the location of the receiving
detectors and the detector output voltages are zero. A knot
in the microwave field will disturb the phase and amplitude
relationships at the knot detectors shaded by the knots. The
signal generated by the detector is proportional to the size
of the knot (amount of knot wood) in front of the detector.
The measurement of the microwave radiation passed through
the lumber is affected by the variations of the attenuation
of the radiation depending on the moisture content, density,
temperature and thickness of the lumber. Also fluctuation of
the transmitted power of the microwave source and variation
of the sensitivity of the microwave detector means causes
changes of the measured signal. Spatial radiation intensity
variations in the measurement field pattern affect the
accuracy of measurement as well.
It is an object of the present invention to provide a
knot measuring system which is self-calibrating and
therefore unaffected by the above mentioned variations.
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The method and apparatus of the present invention include
the use of three microwave transmitting means positioned in
the line of motion of the lumber, The outer and the inter-
mediate transmitting means, which is used to produce a
reference signal, are activated alternatively.
The knot measuring system has an au~omatic gain control
arrangement. To generate the gain control signal, the micro-
wave power is applied to the centre transmitting means,
causing a maximum o~ the field pattern to occur at the
receiving detectors. The gain of the control circuit adjusts
itself to such a value that the detector voltage is ampli~
fied to equal an internal stabilized reference voltage.
Between the transmitting pulses, there is a pause when
the microwave radiation from both the outer and the centre
transmitting means is blocked off. During this time, the
electronic circuit operating by the synchronous detection
principle restores the signal levels to zero.
Further objects and features of the present invention
will become apparent from the following description which is
made with reference to the accompanying drawings in which
Fig. 1 is a block diagram of a flaw detection system
embodying the invention,
Fig. 2 is another embodiment of the invention, and
Figs. 3 and 4 are graphic representations of a signal
produced in the system.
As illustrated in Fig. 1, a piece of lumber 1 being
examined for the detection of flaws, such as for instance
knots, is passed in the line of motion of the lumber between
three microwave transmitters 2, 3 and 4 and to receiving
microwave aerials 5 and 6. A microwave oscillator 7 is
connected to the intermediate microwave transmitting aerial
3 through a SPDT (single pole, double through) switch 8 and
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to the other microwave transmitting aerials 2 and 4 through
the SPDT switch and a T-~unction 9 which causes the xadiat-
ion fields produced by the aerials 2 and 4 to be opposite in
phase. ~he radiation field of all transmitter aerials passed
through the lumber is received by the receiving aerials 5
and 6. The intermediate transmitting aerial and the receiv-
ing aerials are preferably located symmetrically in respect
of the outer transmitting aerials. The received radiation is
measured by means of a detector lO midwa~ in the waveguide
connecting the receiving aerials. The detector which is a
conventional microwave detector diode is connected to a
preamplifier 11. The circuit includes a loop 12 comprising
an automatic gain control amplifier 13 and an integrator 14.
By means of the automatic gain control, the level of the
signal in pulse form from the detector is amplified in such
a way that the mean value of the pulse train equals an in-
ternal stabilized reference value Vref. The integrator 14
transforms the signals to a direct current which controls
the gain of the amplifier 13. A zero set and sampling switch
15 controlled by a control unit 16 which also controls the
SPDT switch 8 couples the signals originating from the outer
transmitter aerials 2 and 4 from the automatic gain
controller 13 to the integrator 17. The base level of the
pulse train from the automatic gain control amplifier 13
corresponding to microwave radiation from none of the
aerials 2, 3 and 4 is clamped to the ground level by means
of the switch 15. When the control unit 16 switches off both
ports of the SPDT switch 8, i.e. when none of the aerials 2,
3 and 4 are radiating and when synchronously the switch 15
is in zero set position, i.e. connected to the ground, a
capacitor 18 disposed before the switch will become charged
to the value Vin and V', the voltage after the capacitor has
the value V. When the switch 15 is switched to the sampling
position, V will start to follow the changes of Vin. During
the sampling period the aerials 2 and 4 radiate and a
capacitor 19 disposed after the switch will become charged
to a voltage prevailing at the time of the sampling. When
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the switch 15 is in zero set position, the capacitor 19 will
maintain this voltage until a new sample will change the
situation. Sampling and automatic calibration of the
measurement data axe repeated e.g. 20.000 times per second.
The intensity of the radiation transmitted to the lumber
from the outer microwave transmitting aerials 2 and 4 is
equal. In case of a knot-free lumber, the radiation passing
through the lumber and received by the receiving aerials,
being opposite in phase, cancel each other at the detector
lO. Zero output is then obtained from the detector 10. A
knot in the radiation field between the transmitting aerial
4 and the receiving aerial 6 or betwe~n the transmitting
aerial 2 and the receiving aerial 5 will cause a chan~e of
the phase difference and the detector 1~ will produce a
signal which is proportional to the size of the knot.
In the embodiment of the invention disclosed in Fig. 2,
parts corresponding with parts shown in Fig. 1 have been
marked with the same reference numerals.
In operation, the embodiment of Fig. 2 operates in the
same way as the system shown in Fig. 1, expect that the
loop 12, the switch 15 and the integrator 17 of Fig. 1 are
substituted by a signal processor 20 which
a) performs an A/D conversion (analog to digital) of the
signals, alternately for V~ which is the voltage of the out-
put signal from the microwave detector when the aerials 2
and 4 radiate and for V~ when the aerial 3 radiates,
b~ stores the conversion results,
c) computes V~
V + V
The control unit 16 transmits to the signal processor
information about the state of the SPDT switch 8.
The two receiving aerials 5 and 6 of Fig. 1 are substitut-
ed by one microwave receiving aerial 21.
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Figs. 3 and 4 show how the output signal from the auto-
matic gain control amplifier in the embodiment of Fig. 1
varies. Vin in Fig. 3 is the voltage before the capacitor 18
and V' in FigO 4 is the voltage after the capacitor. At A
none of the aerials 2, 3 and 4 radiate. At B the aerials 2
and 4 radiate. At C the aerial 3 radiates.
While some specific embodiments of the invention have been
described in detail above, it is to be understood that
various modifications may be made from the spefific details
described without departing from the spirit and scope of the
invention.
